The present invention relates generally to a dental method of bonding acrylic resin to teeth, and more particularly to a dental method of bonding dental composite resin to teeth for restoring them.
So far, restoring composite resin has been bonded to enamel by etching the enamel with phosphoric or citric acid, washing the enamel with water, followed by drying. Treating the enamel even with a bonding material which is composed of a methacrylate ester monomer and a curing agent and has no adhesion to dentine. With this bonding material, a clinically sufficient bonding strength more than 100 kg/cm.sup.2 is obtained between the restoring composite resin and the enamel.
However, no sufficient adhesion is obtained to dentine with the use of a bonding material having no adhesion to the dentine. For instance, Japanese Patent Publication No. 56-33363 and Japanese Patent Publication No. 59-15468 propose to use initiators based on benzoyl peroxide-aromatic tertiary amine-sulfinic acid salt. Even with these initiators, however, no sufficient bonding strength is obtained with respect to the dentine. Various dentine-treating solutions or primers which have been said to have adhesion have also been put forward. For instance, Japanese Patent Publication No. 55-30768 discloses a phosphoric acid ester compound as having adhesion. As measured by the inventors, however, this ester is found to have no such high bonding strength. Also, Japanese Patent Application Laid-Open No. 61-183203, Japanese Patent Application Laid-Open No. 62-2321652, Japanese Patent Application Laid-Open No. 64-90108 and Japanese Patent Application Laid-Open No. 1-279815 disclose surface treating materials which appear to be similar to that disclosed in the present disclosure. However, nowhere in these specifications is described the bonding method envisaged in the present invention, and the inventions disclosed therein is quite different from the present invention in terms of what is intended. The bonding strength measured by the inventors are inferior to that achieved by the present invention.
In Japanese Patent Application Laid-Open No. 54-12338, 4-methacryloxyethyltrimellitic anhydride (hereinafter 4META for short) is disclosed as a functional monomer. Dentine is treated with an aqueous solution of 10% citric acid and 3% ferric chloride and then with a restoring filling material (4META-containing methyl methacrylate/tri-n-butylborane/polymethyl methacrylate), whereby a bonding strength of 12 to 18 MPa is achieved, as set forth in The "Journal of the Japan Society for Dental Apparatus and Materials", 23(61), 29-32, 1982. However, such high strength could not be obtained by the bonding method of the inventors of the present invention. As set forth in The "Journal of the Japan Research Society of Dental Materials and Appliances", Vol. 8 (an extra issue No. 14), pp. 89-90 (1989) and Japanese Patent Application Laid-Open No. 4-126703, on the other hand, it has now been found that initiators based on (thio)barbituric acid derivative/copper compound/chlorine ions are considerably effective for achieving bonding strength with respect to the dentine.
However, the mixed powder/liquid curing type much the same as tri-n-butylborane involves troublesome problems in handling in pre-treating tooth surfaces. Such type is not always sufficient in bonding strength obtained. Thus, there is left much to be improved. Moreover, Japanese Patent Application Laid-Open No. 55-164611 discloses a dental resin composition comprising a carbodiimide compound and an acidic monomer, but this composition has a bonding strength as low as several tens kg/cm.sup.2 and, in this respect, is inferior to currently available dental adhesives.
In view of the present situation where any clinically satisfactory bonding material is not obtained in terms of adhesion to the dentine, the inventors have made studies of how to improve adhesion to the dentine and consequently accomplished the present invention.
According to the present invention accomplished as a result of intensive studies made so as to solve the present problems mentioned above, a bonding method different from the conventional methods in terms of conception has been discovered.
More specifically, a smear layer that is "scum" resulting from the cutting of the teeth is removed by an organic acid incorporated in a treating material and, at the same time, collagenic denaturization of the surfaces of the teeth is inhibited by an iron or copper salt incorporated in the treating material. Then, a primer is applied to the teeth so as to allow a hydrophilic monomer and the organic acid to penetrate deep into the teeth, so that the organic acid can have a chelate bond with respect to calcium that is an inorganic component of the teeth. In addition, the hydrophilic monomer (a methacrylate having a hydroxyl group) bonds to the carboxylic acid and amino group of the activated collagen by way of hydrogen bonds, and this appears to be the reason for the hydrophilic monomer bonding strongly to the dentine. If a carbodiimide compound is additionally incorporated in the bonding material, it is possible to make a strong bond between collagen and the monomer through the dehydrating catalysis of the carbodiimide compound.
This carbodiimide compound is also expected to make a well durable bonding strength, because it causes the carboxylic acid and amino groups in collagen to be bonded together for collagenous crosslinking. In addition, this compound acts similarly on the carboxylic acid residue of the organic acid that is the previously applied primer component. These are considered to cooperate together to allow the bonding material to be bonded to both the inorganic and organic components of the dentine and the bonding chains to be bonded together, thereby achieving a significant improvement in the adhesion strength to the dentine.